On the Forbidden Emission Lines of Iron in Seyfert Galaxies

Abstract

The excitation of the [Fe vr ], [Fe x], and [FE xivj emission lines in Seyfert galaxies is discussed, with special reference to the interpretation of the strengths of these lines in NGC 4151. The [Fe vIr] lines probably arise in the same regions as the [Ne vi lines. Newly calculated collision strengths for [Fe v j are presented, and they are used to find the relative Fe abundance in NGC 4151, approximately N(Fe)/ N(H) = 1.4 X 10- , with an uncertainty of approximately a factor of 2. The [Fe xi and [Fe xivi lines may arise in a high-temperature gas, as Oke and Sargent suggested. On this model the amount of hightemperature gas is calculated by using published collision strengths for [Fe xlvi and published plus estimated collision strengths for [Fe xi. The radiation from this high-temperature gas, if it exists, is energetically sufficient to photoionize much of the "cool" gas in which the other observed emission lines are emitted. Thus the hot gas may be the primary source of photoionization. It may be heated by collisions between clouds moving with velocities corresponding to the observed line widths.